sources tell nbc news it was this man who replaced osama bin laden to head al qaeda that said he wanted to do something big. a massive attack during these, the final days of the muslim month of ramadan. the target wasn't very specific, but the assassins tapped for the job were. al qaeda's most capable branch located in yemen, but they were intercepted leading washington to close 20 posts at least through the weekend and issue travel alerts through august. >> we have to be continually vigilant and have been. >> reporter: today americans were told to leave yemen where al qaeda established a base. >> they have people that are very determined and very patient in attempting to carry out these attacks against the united states. >> reporter: they have especially skilled bomb makers. it was al qaeda in yemen that put an underwear bomb on a flight to detroit and sent a printer cartridge bomb to chicago. it's no surprise the man that took over from bin laden tapped his branch for a big attack. it's been creative, aggressive, and determined to attack the united states. yemeni officials say there w